DR. HECTOR ON PETROLEUM IN NEW ZEALAND.

Southland Times, Volume VII, Issue 523, 20 July 1866, Page 3

DR. HECTOR ON PETROLEUM IN NEW ZE ALAND.

(From tiie iV. 2. Gd&tte) Geological Survey of New Zealand, Wellington, 18th June, 18GG. Sm.— l have the honor to furnish the following information respecting the Hock j oil, or Petroleum, that is found at Tara- j naki, various specimens of which, along with the associated rocks, have been sent to this department for examination. It appears that an exhalation of gas, and bubbles of bituminous matter, have been observed since the earliest days of the settlement, at about half-a-mile from high-water mark, between the main land and Moturoa, the highest of the Sugarloaf islands ; and, according to Dieffen- ! back, was whimsically attributed by the Maoris to the decomposition of an Atna r or spirit, who was drowned there. It was not, however, until November last, that any attempt appears to have been made to search for this oil, by boring or sinking wells on land, and as the.se experiments have to a certain extent proved successful, much, attention has been recently attracted to this natural production, Vhich it is hoped may yet prove a valuable and important .article for export. The petroleum is described as oozing in small drops from cracks and fissures in the rock that forms the Sugar Loaf promontory, but it does not appear that the solid rock itself contains any appreciable traces of oil. All the rock specimens sent are either of the common superficial sand beds of the coast, or of a hard gray rock that proves to be same tracbytic breccia which forms the Sugarl oaves, and which can be traced only a short distance inland towards Mount Egmont. The rock is of volcanic origin, composed of fragments of still older igneous rocks, ejected under the sea in the tertiary period, and cemented together by the fel spathic mud which 'usually accompanies such eruptions. Trachvtic breccia of similar character is of frequent occurrence in New Zealand, encircling the districts where energetic volcanic action once prevailed, either as rudely stratified masses of immense thickness, 'which overlie the deposits that were forming in the sea at the time of their eruption, or as massive dykes that have pierced through and consolidated among these sames strata?. When the nature and origin of this rock is taken into consideration, it is evident that the petroleum it contains can only be accidentally present, and must have been originally derived from some other source ; and, indeed, it is probable that this rock has only acted as a condenser and absorber of gaseous vapor produced by the natural distillation of deepseated strata of carboniferous matter. It should be remarked that among the recent superficial deposits along the same ooast, there are found considerable beds of lignite and decomposing vegetable matter, and though such oV/ sits cannot account for the dissemination of the oil in the rock for a depth of 105 feet (at which depth I understand decided indications of its presence have been obtained), still it is well to bear in mind the existence of such superficial deposits- as it is quite possible that they may give ri?e to small quantities of bituminous oil, and to other indicitions that might tend to mislead explorers in searching for true oil wells. The real source of the oil is most probably to be looked for in the coal seams that belong to the Brown Coal Formation, that is believed to form the base of the series of tertiiry strata that extends under Mount Egmont, and the valleys of the Wanganui and other rivers which enter the sea between Mokau and Otaki. This coal formation, which is probably not a continuous sheet, but occupies isolated depressions in the Paleozoic rocks is overlaid by marine strata of various kinds, comprising clay shales, sandstone?, and limestones that were accumulated during a gradual depression of the land beneath the sea. The volcanic eruptions seemed to have commenced at the period of the greatest depression, n:o.st probably with the eruption of the volcanic rock, like that which forms the trachytic breccia that now yields the petroleum. The volcanic action, at first submarine, was contained with the re-emergence of the land, becoming gradually more feeble and more localised, until the great cone of Mount Egmont was piled up in the atmosphere. The total thickness of the submarine strata, which in the deepest part of the basin overlie the brown coal formation, cannot be less than 2000 feet, and to this must be added at least an equal thickness of submarine volcanic formations, above which rises the true volcaniceone of Mount Egmont to a height of 8270 feet, composed principally of leaves and scoria beds of recent dateUnder this immense accumulation any brown coal beds that exist in the deeper part of the basin must have been subject throughout a lengthened period of time to the combined action of heat from the frequent injection of igneous dykes, anc" moisture from the percolation of water to supply that which was carried off in the form of storm during the volcanic eruptions ; and one of the most probable resuits of the chemical action produced would be the formation of bitumenous vapors that would ascend through the stra-ta along lines occupied by dykes and fissures untiltheyreachedrocks sufficiently cool to cause their condensation into the form of oil. In the foregoing endeavor to account for the remarkable fact of the presence of petroleum in the volcanic rock at Taranaki, it has been necessary to rely greatly on the analogy of geological structure which may r easonably be expected to exist between the district in question and other ports of New Zealand wtiere similar superficial features prevail, as the information which has been derived from actual observation of the distxiet is yet very limited. It must be distinctly understood that there is no similianty between the mode of occurrence of the rock oil, so far as it

}\m yot hiwh found at Tawumln, to that \\\iwh prevails in thrt oil.beariiig districts in tho United HHisa of America and Canada. ; for although it is hold by some geologists that in these countries the oil has also been produced by destructive distillation of coal seams, which are now represented by seams of .anthracite coal, or have been wholly removed by denudation ; still the nature and arrangement of thecondoiisin^rock has besn very different, and this, of course, will completely alter the case so far as the practical search for petroleum is concerned. In the United States the wells are sunk principally in very ancient strata — older, perhaps, than many of the slate rocks of New Zealand, but lying in an undisturbed and nearly horizontal manner over immense areas, j In Oil Creak Valley, according to Professor Draper, these strata consist of clay sh:ile in beds of about one hundred feet in thickness, separated by layers of sandstone of twenty to thirty feet. Tho borings are carried through alternate beds of this description to a depth of 400 fuct before the oil is obtained plentifully, although it also exists in the upper strata in small quantity. In this case it undoubtedly percolates through the more porous layers of sand rock, so that the process for obtaining itis like ordinary well-sinking, and a continued supply can be calculated on with considerable certainty. In Canada the oil is also obtained much in the same way by piercing horizontal beds of limestone and shale of Silurian and Devonian age, the oil being found in the cracks and fissures of the former rocks. In that district there thrre also surface wells sunks in the * supei'ficial gravel and clays that have been saturated with the rising to the surface by natural springsThe steady supply- of rock oil from the American wells is no doubt due to the great extent and regularity of the porous strata in which it has been accumulated, ami through which it percolates in the same manner as water supulyiug artesian wells. The petroleum wells of Italy, Asia Minor, and the Crimea, have more resemblance to what may be expected in New Zealand, in so far that the oil escapes from strata of tertiary age, and is always more or less distinctly connected with active or extinct volcanic agency. Professor Ansted, in a receut article on this subject, describes the petroleum in ! the Crimea as springing from blue clay shales that underlie a crust of marine limestone of recent tertiary formation. The petroleum wells up in the bottom of valleys that have been bored through the limestone, and so exposed the shales, and evidence of deep-seated chemical action is indicated by mud volcanos from which liquid mud, accompanied with an escape of gas, slowly oozes and forms conical mounds and hillocks. In these tertiary strata we have a marked resemblance to the older tertiary rocks of New Zealand which overlie the brown coal formation, and as I have already described, underlie the eruptive rocks of such volcanic centres as Mount Egmont : and lam inclined to think that the proper place to expect petroleum to occur in large quantities may be in con- i nectiou with lines of dislocation at some distance from the centre of disturbance, and where the older tertiary rocks come up to the surface. In the Taranaki district this would be to the north of New Plymouth, and in localities where perhaps there might be no surface indications excepting those which eveiy where mark dislocations of the strata. From the above considerations it is probable that, in the neighborhood of Sugarloaf Point, where these explorations are being made, the boring will have to be carried to a very great depth, before a constant or abundant supply of oil can be looked for ; unless, as is extremely probable, when sinking through a rock which appears to be in parts saturated in oil, an open cavity or fissure be accidentally struck in which a large quantity of oil has accumulated. Such natural cavities are /requently struck when boring in the sandstone in America, the result being a sudden and forcible discharge of gas, oil, or water, ormiddle part of the cavity be first tapped ; but before long this spontaneous overflow always ceases, and then the ordinary method of pumping has to be resorted to. If, however, the oil has been condensed in the fissures that traverse, trachyte breccia at Taranaki, from the form of vapor, and if it does not percolate freely through tha substance of the rock, as it does between the layers 'of sandstone and shale in America, I fear that the pumping will not be of much avail. The specimens of petroleum submitted have been carefully analysed in the laboratory of this department, hy Mr. Skeyyand from the result of his examination, it would appear that the nature and value in relation to other petroleum oils, oi the samples as yet obtained, has been somewhat over-estimated, and that they must only be looked upon as a good indication that really valuable oils may exist in the neighborhood. All the various samples which have been submitted have the same physical characters, having a dirty green color by reflected light and being opakue, except in thin films, when it has a deep red color by transmitted light.